Automatic Transfer Switch (ATS) systems are widely used to control the delivery of power from two different power sources to a load in a variety of situations, both commercial and residential. For example, a private residence normally receives its electrical power from a utility company. For various reasons, however (e.g., location in a region prone to severe weather), the homeowner can desire a backup source of electrical power, so that comfort or at least habitability of the residence can be maintained during periods in which utility power is unavailable.
Typically, a gasoline, diesel, propane or natural gas internal combustion engine-powered electrical generator, capable of generating three-phase power, is installed in or near the residence, and arranged to be connected to one or more of the electrical circuits in the residence in order to provide the desired backup power. However, one cannot simply leave the backup generator permanently connected, in parallel with the utility power, to the residential electrical circuits. Nor can one simply power up a backup generator and connect it to the residential electrical circuits, without first disconnecting the residential circuits from the power lines coming in from the utility.
To effect the proper switching of the residential electrical circuits or other load from the utility to the backup generator (and eventually back again to the utility), transfer switch systems can be employed. While manual transfer switch systems are available, ATS systems have become popular insofar as an ATS system is able to automatically switch from one power source (e.g., the utility) to another power source (e.g., the backup generator) whenever the system detects that the one power source is not properly providing power, without the intervention of a human operator.
Although a generator can provide desired backup power to a commercial or residential site in the case of a utility power failure, there are also situations in which the generator itself might fail. For example, the fuel supply to the generator can become depleted or the generator could experience a mechanical failure. In circumstances where the backup generator experienced a failure, it would be desirable if a secondary, redundant backup generator or other power source could be coupled to provide power to the load at the commercial or residential site.
Despite the need in some circumstances for redundancy in terms of a backup power supply, conventional ATS systems are designed to allow for only two power sources such as a utility and a single backup generator to be alternately coupled to a load. Most situations in which ATS systems have traditionally been used have not been considered to require redundant backup power sources. The market for ATS systems capable of being alternately connected to three or more power sources has historically been small and only recently has been increasing.
Additionally, it has typically been considered that an ATS system capable of being alternately connected to three or more power sources would require a higher level of complexity of internal circuitry, in order to recognize conditions in which each of the three or more power sources should be coupled to the load or decoupled from the load, and appropriately switch the coupling of the different power sources upon recognizing such conditions. Such complexity would increase the price of, and further reduce the market for, such systems. For these reasons, ATS systems capable of being alternately connected to three or more power sources and providing power to a load from any of those three or more power sources simply have not been manufactured.
Given the aforementioned need for ATS systems capable of governing the supply of power from three or more power sources to a load, it would therefore be advantageous if a new ATS system could be devised that allowed three or more power sources (such as a utility, a primary backup generator and one or more secondary backup generators) to be alternately coupled to a load. It would be particularly advantageous if such a new ATS system was not significantly more complicated than conventional ATS systems that allowed only two power sources to be alternately coupled to a load, such that the costs of design and manufacture, and the retail price, of such a system were not excessive. At the same time, it would be desirable if such a new ATS system was capable of operating to determine conditions under which each of the power sources coupled to the ATS system should be coupled to or decoupled from the load, and capable of controlling the coupling and decoupling of the power sources to and from the load accordingly.